fix issue#3: premature banker's queue rotation

- fix premature banker's queue rotation by delaying reverse operation
- add performance tests to demonstrate proper behavior
- improve banker's queue performance by using partial streams
  - move promises to delay.rkt (only banker's queue uses this so far)
  - implement partial streams in partialstream.rkt
  - in banker's queue, use partial stream fns instead of standard streams
add tests for partialstream